The present invention relates to (1) an in-line spectrometric method for determining the distillation residue content in an opaque, dark-colored mixture containing an isocyanate and tar-like materials (hereinafter referred to as an "isocyanate/residue mixture") during the isocyanate production process, (2) the use of the residue content information to control and optimize the distillation process and (3) an apparatus useful for making such residue content determination.
The use of spectral analysis to determine the identity and physical properties of materials is known. Processes in which materials are analyzed during production (i.e., an in-line process) rather than a few hours or days later have long been sought. The advantages of an in-line process are obvious. In-line analysis would eliminate the long turn-around time experienced with traditional lab analysis and would make it possible to optimize the production process by identifying deviations or necessary corrections within a substantially shorter period of time than that which is possible when traditional analytical quality control methods are used.
U.S. Pat. No. 5,151,474 discloses a process control method for a polyolefin polymerization process. In this disclosed process, monomer and co-monomer are added to a flowing stream of solvent at a constant rate. The concentration of monomer and co-monomer in the solvent stream are determined by high resolution multi-wavelength vibrational spectroscopic analysis (e.g., Fourier transform infrared spectroscopy). The rate of addition of the monomer and/or co-monomer is adjusted as needed on the basis of this spectroscopic analysis. Such addition rate adjustments during the production process reduce the variation in product density.
U.S. Pat. No. 5,153,140 discloses a process for controlling and optimizing industrial processes for the production of dyes, fluorescent whitening agents and their intermediates. In this process, differential analysis of the UV/VIS absorption spectra of at least one starting compound and at least one reaction product are used to monitor and control the production process.
U.S. Pat. Nos. 5,206,701 and 5,223,715 disclose specially designed spectrophotometric apparatus for quantifying the physical properties of hydrocarbons, crude petroleum and other black oils. In these specially designed apparatus, a sample cell or probe may be inserted directly into the sample source. The spectral data generated are related to the physical properties of the product by means of chemometric models.
These prior art processes do not, however, disclose a method for process control in which the concentration of tar-like by-products generated during the production process in the distillation residue may be monitored using standard, commercially available equipment. Monitoring the residue by-product content rather than amount of reactant material(s) or reaction product is advantageous because product loss is minimized and formation of a large concentration of residue that could cause processing problems is avoided. However, because the residue of a distillation process is generally opaque and dark in color and more varied in composition than the desired product or the reactants, it is more difficult to monitor such residue by an in-line process.
An in-line spectrometric process for monitoring a residue that generates the desired analyses with a high degree of accuracy within a very short period of time using standard, commercially available equipment would therefore be advantageous.